1. Field of the Invention
The present invention relates to a solar cell, and, more particularly, to a solar cell having semiconductor layers with multi-quantum wells that transition from a small band gap energy state to a large band gap energy state, and a method of manufacture.
2. Description of the Prior Art
Satellites and space stations require light weight, high power solar cells. The conventional solar cell comprises a p-n junction formed from a crystalline semiconductor material, which determines a single band gap energy state. When photons having an energy state greater than the band gap energy state are incident on the solar cell, electrons at the p-n junction absorb the energy and move to a higher energy state. This produces electron-hole pairs. An electromagnetic field produced by the voltage associated with the p-n junction causes the electron-hole pairs to move, thus producing a photocurrent. Accordingly, the solar cell converts the incident light energy into electrical energy.
Typically the solar cell is formed from crystalline silicon and has a deep junction. Such a solar cell is extremely responsive to longer wavelength light and exhibits much less response to shorter wavelength light. It achieves an efficiency of approximately 18%. However, in outer space shorter wavelength light predominates. Hence, in another embodiment the single p-n junctions are made closer to the surface and thinner to enable the solar cell to respond more efficiently to the shorter wavelength light. In still other embodiments, solar cells are made with double junctions and from crystalline gallium arsenide and gallium indium arsenide in attempts to create more efficient energy converters. A triple junction gallium arsenide device was recently developed by Spectrolab Inc. and is alleged to have an efficiency of 26.8%.
What is needed, therefore, is a solar cell that efficiently converts more light of all wavelengths in the light spectrum, or photonic energy, into electrical energy and is relatively light weight. In addition, it is desirable to develop a process for manufacturing this more efficient solar cell.
The preceding and other shortcomings of the prior art are addressed and overcome by the present invention which provides generally, in a first aspect, a solar cell comprising a superstrate material that is transparent to light, coated with a plurality of layers formed from semiconductor materials characterized by multi-quantum wells and multiple band gaps. The first band gap has the smallest, and the last layer has the largest band gap, and the intermediate layers have band gaps transitioning from the smallest to the largest. The overlapping band tails between adjacent layers eliminate the need for a tunneling junction. Also, the use of amorphous materials is advantageous because they are less susceptible to degradation upon long term exposure to radiation.
In another aspect, the present invention provides a two dimensional electron gas interface layer in addition to the regular p-n junction. In this case, p and n layer interfaces are highly doped with silicon (delta doped) to form atomic layers that saturate with electrons to reduce the interface electron-hole recombination.
In yet another aspect, the present invention provides a method of fabricating a solar cell comprising: providing a glass superstrate having a layer of conductive transparent oxide, and forming a plurality of layers of semiconductive materials on that oxide, the materials characterized by multi-quantum wells and multiple band gaps, the layers transitioning from a first layer formed from a material having the smallest band gap, such as indium antimonide, to a last layer formed from a material having the largest band gap energy state, such as indium arsenide, indium gallium arsenide, indium phosphide, indium gallium arsenide phosphide, gallium arsenide or silicon.